Noise reduction circuit for sound recording



Dec. 17, 1940. B, KREUZER 2,224,914

NOISE REDUCTION CIRCUIT FOR SOUND RECORDING Filed April 29, 1939 2Sheets-Sheet l jA/PUT l dL 774 as G42 VANOMA'TER SHurrEe srze-cr/ou v 25/1/2772 Zea/2E2, W;' a W%@ M Dec. 17, 1940.

NOISE REDUCTION CIRCUIT FOR SOUND RECORDING 7215s; 34AM!) 36' 6x10VOLTAGE .z/vsaanp 34:0

B. KREUZER Filed April 29. 1939 2 Sheets-Sheet 2 Patented Dec. 17, 1940.

PATENT. OFFICE 7 NOISE REDUCTION cmomr FOR'S'OUND RECORDING BartonKreuzer, Los Angeles, Calif assignor to Radio Corporation of DelawareAmerica, a corporation of Application April 29,1939, Serial No. 270,87611 Claims. [wins-100.3)

This invention relates to sound recording systems and particularly tosound recording systems wherein noise reduction is applied during therecording.

Sound recording systems of the variable area and variable density typesare well known in the art. In the area type of system the film islightimpressed with a beam varying transverselyof the sound trackportion of the film, while in the density type, the film islight-impressed with a beam of constant length and width, but of varyingintensity. The application of noise reduction to both types of systemsis also well known wherein all or substantially all of the unmodul-atedtrack area is caused to be opaque in the final print produced by arearecording, or the modulation of the variable intensity beam is such thatthe density of the final print varies between a point at one end of thecharacteristic curve and a point higher up on this curve at the highermodulations.

The present invention is particularly directed to an optimum practicalmethod of and apparatus for applying the noise reduction action.Referring specifically to the sound recording type of system whichproduces a variable area duplex track, two narrow, transparent tracesare found in the final print at times of no signal, the outer portionsbeing under control of noise reduction shutters and the central portionbeing under control of the movements of the galvanometer mirror. Thenarrow traces could be eliminated but are provided to prevent filmbreathing. In this type of system, the noise reduction shutters areactuated with direct current obtained from rectifying a portion of thesignal being impressed on the galvanometer. Thus, actuation of theshutters occurs substantially simultaneously with the actuation of thegalvanometer. Therefore, and particularly when sounds which increaserapidly in amplitude are impressed on the galvanometer, the shutters mayfail to clear the track area sufficiently to accommodate the peaks ofthe modulations, and so-called clipping of the peaks occurs. 7

On the other hand, if the shutters were actuated with sufficientrapidity to clear the track area for all types of applied sounds, thetrace or traces made by the shutters would introduce audible soundscommonly known as plops or swishes. Howeveigif the traces referred toabove are wide enough initially to accommodate sounds within a certainamplitude range, then the transparent areas at times of no signal wouldintroduce ground-noise which is undesirable.

Thus, the present invention is a noise reduction system having an actionwhich will reduce both ground noise at times of no signal andpeakclipping to a practical minimum. An object of the invention,therefore, is toimprove the noise reduction action of the noisereduction portion of a sound recording system.

Another object of the invention is to control the noise reductionportion of the apparatus to f produce a soundrecord in .which the lighttransmission therethrough is at an optimum minimum during times of nosignal.

A further object of the invention is to actuate the noise reductionportion of the system at different speeds depending upon the amplitudeof the signal being recorded.

A further object of the invention is to automatically actuate the noisereduction shutters at different rates according to the amplitude of thesignal.

A further object of the invention is to vary the rate of application ofnoise reduction in accordance with the amplitude of the signal.

Afurther object of the invention is to produce an anti-groundnoise soundrecord in which the average transparency varies with amplitude of themodulations thereon.

Although the novel features which are believed to be characteristic ofthis invention are pointed out with particularity in the claims appendedherewith, the manner of its organization and the mode of its operationwill be better understood by referring to the following description readin conjunction with the accompanying drawings forming a part thereof, inwhich Fig. l is a diagrammatic representation of a variablearea type ofsound recording system with a schematic circuit embodying the invention;

Fig.2 is a graph illustrating the operation of the system shown in Fig.l;

Fig. 3 is ,a sound track record resulting from the operation of thesystem of Fig. 1;

Figs. 4, 5 and 6 are characteristic curves of the various tubes employedin Fig. 1; and Fig. 7 is a schematic diagram of another embodiment ofthe invention.

Referring now to Fig. 1, the right-hand portionof the drawingdiagrammatically represents a variable area recording system of thestandard duplex record type such as that disclosedby U. S. Patents2,102,776; 2,102,777 and 2,102,778. Light from a light source 5 iscollected by a lensiprojected through an aperture plate 1 withanappropriate aperture Bftherein, from "which it' is projected throughlens ID to a mirror ll of a galvanometer having an actuating coil I 2. Apair of shutters l4 operated by a coil I5 control the amount of lightpassing through the ends of the aperture 3. Reflected light from thegalvanometer mirror I l is projected to a slit mask I! having a slit 18therein, from which it is collected by objective lenses I9 and impressedupon the sound track portion 20 of a film 2i. The abovedescribedapparatus is the usual recording system for producing standard variablearea duplex tracks.

The sound to be recorded is detected by a microphone 24 and translatedinto electrical currents which are amplified by an amplifier 25 and thenimpressed upon the coil l2 of the galvanometer over conductors 21. Theimpression of these currents on the galvanometer causes the mirror H tofollow the instantaneous values of the amplitude and frequencies of thesignal to vibrate the light beam normal to the slit IS. A portion of thecurrent output of amplifier 25 is impressed through a transformer 32upon a rectifier 28 having a time filter circuit consisting of a seriesresistance 29, a shunt resistance 30 and a shunt capacitance 3|. Thistiming filter circuit is disclosed and claimed in my Patent 1,999,- 700,issued April 30, 1935,

The rectified output of rectifier 23, after passing through the filter,is impressed directly upon the grids of three triode vacuum tubes v34,35 and 36, tubes 34 and 35 having their outputs connected in parallel tofeed the actuating winding [5 of shutters l4 through a resistance 3! andpotential supply 38. The plate of tube 33 is directly coupled to thegrid of tube 49, plate potential for the tube 36 and grid bias potentialfor the tube 43 being obtained over a common resistance 42 which islarge compared with the plate resistance of tube 39. The resistance 42is connected near the positive end of a potentiometer resistance 43supplied from the potential source 38. The grid bias on tube 35 is madedifferent from that .on the grid of tube 49 by connecting the respectivefilaments of the tubes 36 and 40 to different points on commonpotentiometer 43, the points of connection being made variable topermitadjustment of the bias. The plate of tube 43 is connected to thenegative end of resistance3l, this resistance being large with respectto the plate resistance of tube 43. Grid bias for tubes 34 and 35 isobtained from a potentiometer 45 connected across apotential source 46.The bias on the grid of tube 36 is made different from that on the gridsof tubes 34 and 35 by the connecting of the cathode of tube 36 topotentiometer 43, while the cathodes of tubes 34and 35 are connected toground.

The above noise reduction circuit just described may produce differentactions of the shutters l4, one preferred action being illustrated inthe graph of Fig. 2, wherein the input voltage, for instance, toamplifier 25 is plotted against galvanometer and shutter deflections.The curve A represents the galvanometer deflection for a certain rangeof input voltages, this curve showing a linear relationship betweenthese two factors, maximum input voltage corresponding to full-track ormodulation being shown at point B. The curve C of Fig. 2 represents theshutter deflection for substantially the same range of input voltages,it being observed that in the lower range of input voltages, theshuttersseparate very rapidly as the voltage increases, while this speed ofshutter deflection becomes much less rapid with further increases in theinput voltage until the shutter action becomes substantially the same asthat of the galvanometer. As shown on the graph, the shutters lead thegalvanometer with respect to the track area so that the shutterscompletely clear the track area at a level of 3 db. below the amplituderequired for 100% modulation of the track. This type of. action providesa rapidity of shutter action necessary to prevent undue clipping of thesound modulations at the beginning of the sounds and preventsintroducing deleterious audible sounds by the traces formed by theshutters. At the same time, the optimum amount of noise reduction may beapplied at times of no signal.

An illustrativesound record resulting from this operation is shown inFig. 3, wherein the opaque areas D are produced by the noise reductionshutters, and the opaque area E, by the position of the light beam onthe slit it of the mask l1. As the galvanometer mirror is actuated, theopaque area E is varied in accordance with the instantaneous values ofthe sound, and a sound track is produced as shown by the traces F, thefilm being advanced during the original recording in the direction ofthe arrow. This illustrative sound record also shows the action of theshutters during the impression of the signal on the galvanometer, and itwill be observed that the separation speed of the shutters in the lowerrange of amplitude is more rapid than at the a higher amplitudes toproduce the curved noise reduction shutter traces G. The closing time,however, is slower than the opening time, as shown by the noisereduction traces H. This timing relation is caused by the action offilter 29, '30 and 3!, as explained in my Patent 1,999,700, issued April30, 1935, referred to above.

The present invention is particularly directed to the operation of thecircuit consisting of elements from 34 to 46, particularly tubes 34, 35,36 and 40. The shutters M are maintained at their minimum distance ofseparation by an initial current flow from the tubes 34 and 35 throughwinding l5 and resistance 31, the amount of this current flow beingdetermined by the bias on the grids'of tubes 34 and 35 in accordancewith the setting or" the potentiometer 45. This adjustment i'smade suchthat the narrow zero traces between opaque areas D and E, shown in Fig.3, areproduced by very narrow light beams passing to the filmat times ofno signal. The initial adjustment'of the grid bias of tubes 38 and 40 issuch that no output current flows from the tube 40 through theresistance 31 at times of no signal.

To more clearly illustrate this condition, reference is made to Figs. 4,5 and 6, the points a: on the curves representing the static bias or nosignal adjustment of the different tubes. For tubes 34 and 35, the point:c is well up on the curve, thus providing the current required formaintaining the shutters [4 at their minimum separation. Tube 36 has alarger bias (see Fig.

5), while'tube 40 has a bias which substantially prevents any outputcurrent through resistance 31 (see Fig. 6).

Now, as the signal is rectified, the bias on tubes 34, 35 and 3B isincreased by the increase in rectified current, and the output currentfrom tubes 34 and 35 flowing through the winding l5 and resistance 31'decreases as shown by point y in Fig. 4. 'I'hisvcauses a certain amountof separation of the shutters I-4. Simultaneously, however, the-biasvoltageis reduced on the grid of tube 36,-as shownby point 1/ in Fig.5,-which from flowing decreases the plate current from tube 36 and thusraises its plate voltage; Since this plate voltage is also the gridbias'voltage for tube 40, the grid voltage for tube 40 has now been made1 more positive, causing tube 46 to draw current through resistance 31(see point y in Fig. 6'). This current, through the resistance 31,produces a voltage drop in this resistance which decreases the platecurrent from tubes 34 and 35 and thus also reduces the currentin thecoil l5. Therefore, as the'rectifier' 28 begins to. function with signalinput, there are two factors which reduce the current in coil l5, onebeing the normal reduction in the plate current of tubes 34 and 35caused by the increase in the grid bias thereof, and the other, thereductionin the plate current of these tubes by-the reduction of theirplate voltage caused bythe output current. from tube 40., The result ofthe action just described produces the steep portion of the curve CinFig. 2 and the steep portion of the traces G in Fig. 3.

Now, as the output of. rectifier 28 continues to increase in accordancewith the signal input, the current from tubes 34, 35 and 36 continuestodecrease as their grid biasesincrease, as shown by Figs. 4 and 5. Whenthis .bias has .reached point 2, it will be observed that tubev 36 hasreached cutoff, and any further increase in its bias does not alter itsplate current. Thus, the output current from tube 40 becomessubstantially constant at point z in Fig. 6, and further increases inoutput of rectifier 28 ,do not affect the plate current of tube. 40 orthe plate voltage of tubes 34 and Hand, consequently, have no effect onthe separation of shutters 14. Because of the curvature of the toe ofcharacteristics of tubes 36 and 46, the transition point where thecumulative action ceases is, gradual, thus producing thecurved traces Gin Fig. 3. After the tubes 36 and 40 cease to function, only'thedecrease in bias on tubes 34and 35 has any efiect upon decreasing thecurrent in coil I5, and consequently the shutters separate more or lesslinearly with, respect to input voltage. This action is shown in Fig. 2by the straight portion of curve C.

Thus, by the action of the circuit arrangement shown, including tubes34,35, 36 and 40, a very rapid movement of the shutters occurs over thelow amplitude range of the signal, and amore linear relationship betweeninput and shutter de-. flection is obtained after a certain amplitude isreached. It is to be realized that although two tubes 34 and 35 areshown in parallel, a single tube may also be used; and, also, instead oftriode tubes 34, 35, 36 and 40, exponential or variable mu tubes aresuitable. Furthermore, an oppositely phased action is obtainable byusing an intermediate tube to obtain a decrease in bias on tube 36 withan increase in signal amplitude which will decrease the output currentof tube 46 from a certain initial value.

This type of circuit shown in Fig. 1 thus provides a very flexiblemethod of obtaining certain types of characteristic shutter, openingswith respect to certain types of signal, such as dialogue and differentmusical compositions. That is, some signals may be ,of substantiallyuniform amplitude throughout, while others are made up of periodicsequences of high amplitudes, and different bias settings for the tubesmay be quickly made to provide the optimum shutter action for theparticular type of signal being recorded.

Referring now to Fig. 7, there is shown a circuit modification of thenoise reduction rectifier and amplifiercircuit 'of Fig. 1. In theoperation of this circuit, the cumulative action between two rectifiersis in phase opposition over the upper range of signal amplitudes toproduce a shutter action similar to curve C of Fig. 2. The input to thiscircuit may be from the amplifier 25 of Fig. 1, the output of which isdivided, one portion being impressed upon an amplifier'50. The output ofamplifier 50 is impressed upon a rectifier 5|, in the output of which isa timing filter, circuit including a series resistance 52, a shuntresistance. 53" and a condenser 54. This timing circuit-isconnected tothe input of a direct current amplifier 56, the output of which feedsthe noise reduction winding I5 of Fig. 1.

The other portion of the output of amplifier 25 is impressed directlyupon a rectifier 58 having a timing filter circuit 59, 60 and BI in itsoutput circuit. A source of grid bias for amplifier tube 56 is shown asa potential source 63 shunted by potentiometer 64. A biasing circuit forrectifier 58 is also employed, this biasing circuit comprising aresistance 66 shunted by a potential source 61. The essentialdifferences, therefore, between the two parallel rectifierbranchesfeeding the direct current amplifier 56 is that a preamplifier is in theinput of rectifier 5|, while a bias is applied to the rectifier 58.These diiferences produce a shutter action similar to curve C of Fig. 2,as mentioned above, as follows:

The inputcircuit of rectifier 5| is impressed with a signal potentialdepending upon the gain of amplifier 56 which produces an increase inthe grid bias of tube 56, as explained in connection with Fig. 1. Thisincrease in bias decreases the current through winding l5 and allows theshuttersto open rapidly with respect togalvanometer deflection. The gainof amplifier 50 and the setting of the timing filter circuit 52, 53, and54 controlsthe speed of this action. However, the same signal isimpressed upon rectifier 58, but at a lower level because no similaramplifier to amplifier 56 is used in its input circuit. Also, there is astatic bias applied to the rectifier 58 at 66 so that it will not drawcurrent until a certain amplitude of input signal is reached. Theadjustment of this rectifier bias determines the position of the bend incurve C of Fig. 2, since the voutputfrom rectifier 58, when itfunctions, produces a voltage across resistance 66 which opposes orbucks the voltage across resistance 53 caused by the output of rectifier5|. (See polarities' of resistances 53 and 60 and connections thereof.)Thus, the rapid shutter action caused by rectifier 5! alone is sloweddown at the higher amplitudes of the signal, the resulting action beingdependent upon the difierential voltage across 53 and 60, which issubstantially proportional to the gain of amplifier 50. The linearportion of curve C is thus produced. This parallel rectifierarrangement, therefore, will provide the shutters'l4 of Fig. 1 with adeflection similar to thatshown in Fig. 2 by curve 0.

Although the invention has been described above embodied in a variablearea duplex recording system, it is also applicable to the singleshutter type-of noise reduction system as well as the type of systemwhich does not employ shutters, but biases the galvanometer or otherlight modulating means directly as shown in U. S. Patents 1,854,159 and1,888,724. It will also be understood that the circuit may be used tovary the modulation point of a light beam so that the variable densityprint is substantially opaque at times of no signal and the modulationpoint varice between a point at one end of the characteristic and apoint higher up on the characteristic at higher modulations.

I claim:

1. A circuit comprising means for generating alternating currents, meansfor rectifying a portion of said currents, means for utilizing a portionof said alternating currents in direct proportion to the variations inamplitude of said alternating currents, means for utilizing saidrectified portion of said alternating currents, and means interposedbetween said last-mentioned means and said rectifying means, saidlast-mentioned means including a plurality of electronic elementsselectively operable with the variations in amplitude of saidalternating currents to vary the proportionality between the amplitudevariations of said alternating currents as impressed on said rectifyingmeans and the amplitude variations of said rectified currents asimpressed on said rectified current utilizing means.

2. A circuit comprising a source of alternating currents varying inamplitude, means for directly utilizing a portion of said alternatingcurrents, means for rectifying another portion of said alternatingcurrents, means for utilizing said rectified currents, and a pluralityof electronic means selectively operative with amplitude for varying thereaction produced by said rectified currents with respect to thereaction produced by said alternating currents.

3. A circuit comprising a source of alternating currents varying inamplitude, means for directly utilizing a portion of said alternatingcurrents, means for rectifying another portion of said alternatingcurrents, means for utilizing said rectified currents, and means forvarying the reaction produced by said rectified currents with respect tothe reaction produced by said alternating currents, said last-mentionedmeans comprising a plurality of vacuum tubes, all of which areeffectiveover a certain range of amplitudes of said alternating currents and onlycertain others of which are effective over another range of amplitudesof said alternating currents.

l. A circuit comprising a source of alternating currents, a rectifierfor said currents, a direct current amplifier for said rectifiedcurrents, a load in the plate circuit of said direct current amplifier,and a second direct current amplifier having an initial bias diiierentfrom said first direct current amplifier, said second direct currentamplifier and said first direct current amplifier cumulatively effectingsaid load over the lower range of amplitudes of said alternatingcurrents and only said first direct current amplifier operating over thehigher range of amplitudes of said alternating currents.

5. A sound recording system comprising a source of alternating currentsto be recorded, a light beam, means for directly modulating said beamwith a portion of said alternating currents, and means for modulatingsaid light beam with the average value of said currents, saidlastmentioned means comprising a rectifier and a plurality of directcurrent amplifiers interposed between said source of alternatingcurrents and said second-mentioned modulating means, all of said directcurrent amplifiers operating cumulatively on said second-mentionedmodulating means over a certain range of amplitudes of said alternatingcurrents, while only certain others of said direct current amplifiersoperate on said second-mentioned modulating means over another range ofamplitudes of said alternating currents.

6. A direct current amplifier circuit comprising a plurality of vacuumtubes, a common input circuit for said tubes, a pair of output circuitsfor said tubes, means for impressing variations in voltages on saidinput circuit to produce variations in current in said output circuits,one of said output circuits comprising an inductance and resistanceconnected in series and the other of said output circuits comprisingsaid resistance, and means for difierently biasing said vacuum tubes,the plate voltage on one of said amplifiers being the grid bias onanother of said amplifiers.

7. A rectifier circuit comprising a source of alternating current, apair of rectifiers, means for impressing a portion of said alternatingcurrents directly on one of said rectifiers, means for amplifyinganother portion of said alternating currents, means for impressing saidamplified alternating currents on another of said rectifiers, means forconnecting the outputs of said rectifiers in phase opposition, a directcurrent amplifier, and means for impressing the differential output ofsaid rectifiers upon said direct current amplifier.

8. A rectifier circuit in accordance with claim '7 in which saidfirst-mentioned rectifier is provided with a predetermined bias.

9. A rectifier system comprising a source of alternating currents, apair of rectifiers for said currents having a common input circuit,means for impressing said alternating currents on said input circuit, acommon output circuit for said rectifiers, means for varying the time ofoperation of one of said rectifiers with respect to the other of therectifiers, and means for combining the output of said rectifiers.

10. A direct current amplifier circuit comprising a plurality of vacuumtubes, a common input circuit for said tubes, a pair of output circuitsfor said tubes, means for impressing variations in voltages on saidinput circuit to produce variations in current in said output circuits,one of said output circuits including a load circuit and an impedanceconnected in series and the other of said output circuits including saidimpedance, and means for difierently biasing said vacuum tubes forselective operation in accordance with the amplitude of said inputvoltages.

11. A rectifier circuit comprising a source of alternating current, apair of rectifiers, means for impressing a portion of said alternatingcurrents directly on one of said rectifiers, means for amplifyinganother portion of said alternating currents, means for impressing saidamplified alternating currents on another of said recti fiers, and meansfor connecting the outputs of said rectifiers in phase opposition.

BARTON KREUZER.

